In a known wireless network, such as a telecommunication network, the User Equipment (UE) is able to estimate a quality of a received signal based on radio measurements. Since the quality of the received signal changes rapidly, it is preferred to quickly adapt modulation and coding of signals, to be sent from a Base Station (BS) to the UE, to changes in the quality of the received signal in order to benefit from the—still sufficiently accurate—radio measurements. Not only should the modulation and coding be adapted to the quality, e.g. channel conditions, but modulation and coding should additionally, or alternatively, be adapted to requirements concerning Quality Of Service (QoS) for data transmitted between the UE and the BS.
For some type of data, high throughput should be prioritized over e.g. delay. Then, for this type of data a Block Error Rate (BLER) should be set such that an average throughput it optimized, or nearly optimized. In contrast thereto, for e.g. delay sensitive data, it would be obvious to prioritize low delay over high throughput. Accordingly, the BLER should be targeted to be low BLER. In this manner, time consuming retransmissions are likely to be avoided.
In general, some Modulation and Coding Scheme (MCS) might perform better than some other MCS under a certain radio condition, e.g. quality of the received signal. It is therefore preferred to adapt the MCS to the certain radio condition, e.g. given by the quality of the received signal, to improve performance of the transmission of data. To adapt the MCS to the certain radio condition is generally referred to as link adaptation. Link adaptation means that transmission parameters, e.g. defining a specific MSC, are set depending on the radio conditions. As an example, Long Term Evolution (LTE) supports 29 different MCS, given by respective transmission parameters, where the different MCS will impact data rate of the transmission of data. A higher value of MCS implies higher data per symbol ratio than a lower value of MCS. Therefore, the higher value of MCS allows for a higher data rate as compared to the lower value of MCS. A disadvantage with setting the MCS to the high value is that the signal will be more sensitive to noise and/or interference.
In essence, link adaptation is a way to adapt to the signals transmitted, e.g. by the BS, depending on feedback, such as the measurements of quality mentioned above, from the UE. Due to a random nature of the quality of the received signals and noise at a receiver, e.g. the UE, the link adaption may not be perfect. This means that a report of the quality, e.g. from the UE to the BS, may not always create perfect link adaptation, i.e. an optical choice of MCS. The report may be given by the UE in terms of a Channel Quality Indicator (CQI) report. The CQI report may encode an observed radio condition measurement such as a Signal to Interference and Noise Ratio (SINR). Alternatively, the CQI report may encode an MCS, i.e. an estimated appropriate MCS by the UE considered to be a candidate MCS for communication to the UE.
In order to compensate for this imperfection, a so called outer loop link adaptation is typically used. The outer loop link adaptation is a method that adapts the MCS depending on acknowledgement(s) in Hybrid Automatic Repeat reQuest (HARQ) report. In LTE, the UE sends HARQ reports to the BS to request, or not request, retransmission of erroneously received data transmission(s). In outer loop link adaptation, a fixed BLER of 10% is a typical target value. This means that 90% of the data transmissions, such as transport blocks, are transmitted successfully. Note however that an effective BLER typically varies and that the target value for BLER depends on e.g. the type of data as discussed above.
As mentioned above, the link adaptation sets the MCS based on the feedback from the UE. The feedback of positive acknowledgment (ACK) messages notifies the BS of a correctly received packet and drives the MCS towards more optimistic link adaptation thresholds. On the other hand, negative acknowledgment (NACK) messages drive the offset towards more conservative link adaptation thresholds. This feedback could as explained above be misleading since randomness of the actual quality and noise at the receiver creates an uncertainty of what MCS to use at the BS. A difference between the CQI reported by the UE and a “true” CQI is defined as a CQI offset. The “true” CQI is thought of as being free from any error sources in the quality estimation performed by the UE. Some UE(s) do frequently report a bad estimate of their CQI, for example UEs with exposed to a high levels of noise.
An example illustrating the outer loop link adaptation method is depicted in FIGS. 1, 2a and 2b. In FIG. 1, an offset, such as the CQI offset, is plotted as a function of time for a first simulation and a second simulation, both with a BLER target of 10%.
In FIG. 1, an offset from the first simulation, relating to a first UE, is illustrated as a solid line and an offset from the second simulation, relating to a second UE, is illustrated as a dashed line. Both the first and second simulation illustrates adjustment of an offset directly after an establishment of a radio session, e.g. during attach for LTE.
FIG. 2a illustrates HARQ feedback received by the BS in the first simulation. As long as “Is Ack” is 1, or true, no NACK has been received. When “Is Ack” is true for a greater number of data transmissions, such as transport blocks, than expected compared to the BLER target, it may be deduced that the reported CQI is underestimated/inaccurate, and the offset is adapted accordingly. From FIG. 2a, it may be seen that, in the first simulation, the first NACK arrives after a first time period T1, e.g. 0.1 s or less. FIG. 1 shows that during this first time period T1, the CQI offset decreases. (The absolute value of the offset increases since the offset is negative in this case.) The NACK feedback received at T1 triggers an increase of the offset. The offset then turns back and forth to become relatively stable towards an—at least momentary—equilibrium (solid bold line). In this first simulation, the reported CQI is fairly accurate, i.e. the reported CQI generates a choice of MCS that is fairly close to the BLER target (within certain margins). In the first simulation, the offset becomes approximately OF1 dB, such as −0.5 dB. That is, the reported CQI is roughly OF1 dB less than the CQI which had given the target BLER directly after network establishment
The second simulation shows that, during a second time period T2, e.g. 0.3 s or more, the CQI offset decreases also for the second user equipment before it goes towards a relatively stable value (dashed bold line). In this simulation, the reported CQI is misleading, i.e. the reported CQI generates a choice of MCS that does not come close to the BLER target (within certain margins). As seen in FIG. 2b, the first NACK in this simulation does not arrive until after the second time period T2. Therefore, the reported CQI is more heavily underestimated/inaccurate than in the first simulation. The offset reaches steady state at approximately OF2 dB, such as −3.5 dB, for the second simulation.
In view of the above, a problem is how to reduce time to reach steady state of offset estimation in outer loop link adaptation.
WO2013025131 discloses a method relating to handover of a mobile station from a source cell to a target cell in a cellular network. With the method initial communication between the target cell and the mobile station is improved thanks to that communication improving data between the source cell and the mobile station is collected prior to handover. The communication improving data, such as Channel Quality Indicator, Precoding Matric Indicator (PMI), power headroom etc., may thus improve communication at a new connection towards the target cell.
WO2013025131 addresses how to handle communication improving data for the case of connected mode devices during the handover. A disadvantage with WO2013025131 is that the solution is only applicable for connected mode devices.